This invention relates to an improved direct fired power cycle which generates and employs a combustion gas, including carbon dioxide, as a working fluid and an indirect power cycle employing carbon dioxide in separate and distinct combustion and working fluid loops and in particular to such cycles wherein the carbon dioxide working fluid is maintained at all times in a gaseous or quasi-liquid supercritical phase. Presently, electrical energy is generated primarily by indirect firing methods. A mixture of fuel and air is burned to generate high temperature combustion gases. These gases transfer heat via heat exchange to water in a boiler, thereby boiling the water at high pressure and temperature to provide superheated steam. This steam is expanded through a steam turbine to a very low pressure thereby generating shaft power which is converted to electrical energy.
Such methods of power generation are far from optimally efficient. Where water/steam is utilized as the working fluid, extremely low pressures (e.g. below 1-2 inches Hg) must be employed in order to condense the fluid from a gaseous to liquid form for recycled use. Further large amounts of latent heat are required for the water to undergo the required phase changes. nsequently, power cycle efficiencies are typically less then 38%. Such inefficiency is compounded by several factors: Because of material constraints of high pressure (e.g. 2000-3000 psi) equipment such as boilers used by indirect systems, steam temperature must be kept below 1200.degree. F. Because higher temperatures enhance cycle efficiency, such efficiency is thus lost in low temperature indirect systems.
The very large expansion ratios exhibited by present systems additionally necessitate the use of large expensive machinery (e.g. steam turbines). The low rate of heat transfer from hot combustion gas to water in the boiler requires that enormous combustion chambers be provided.
Present power systems (both direct and indirect) often use freely available air for combustion. However, large quantities of inert nitrogen are present in the air. Such nitrogen is useless for the generation of power (e.g. for use in the working fluid) and furthermore causes pollutants (nitric oxides) to be produced. Such pollutants must be removed thereby adding considerably to cost of the system.
Utilizing oxygen alone rather than air in the combustion step of a power cycle eliminates nitrogen from the power cycle and greatly reduces the size of equipment required by the system. For example, a high pressure (200 atm) oxygen burner requires a volume 703 times smaller than an ambient pressure air burner. However, oxygen firing is typically prohibitively expensive particularly as compared to "free" air. Very expensive materials of construction must also be utilized to accommodate the high flame temperatures generated in oxygen firing. In fact, the combustion gases produced in oxygen fired systems must be diluted with an inert medium so that such high temperatures may be moderated to levels which enthalpy recovery devices (boilers, turbines) require.
A direct fired power cycle which substitutes recycled carbon dioxide combustion product for a certain portion of the oxygen is known. U.S. Pat. No. 3,736,745. However, that system teaches maintaining the carbon dioxide working fluid above a critical pressure of 1100 psi at all points in the power cycle. At certain points, the carbon dioxide is converted far below its liquid gas interface (e.g. the CO.sub.2 dome) to a liquid phase. An undesirably large amount of latent heat is lost and a significant amount of work is required to return to a gaseous phase. Further, because present materials constraints limit operation to 3000 psi, the maximum available pressure ratio is limited to 3000/1100 thus severely limiting available turbine work. Consequently, optimal efficiency is not achieved.